Endocranial Imprints of Lucy Indicate Extended Childhood

New research headed by paleoanthropologists Philipp Gunz and Simon Neubauer has revealed that the Australopithecus afarensis, also known as Lucy's species, had a brain similar to that of an ape. Extended brain growth in the species also suggests that infants may have depended on caregivers.

The new research from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, has studied the Australopithecus afarensis species, who have inhabited East Africa millions of years ago and are widely considered as ancestors of all later hominins, which include humans.

Zeresenay Alemseged, senior author and field director of the Dikika project that discovered a skeleton of an Australopithecus child in 2000 in Ethiopia, explained that Lucy and her kind walked upright, used sharp stone tools, and had brains that were 20 percent larger than chimpanzees.

Currently, the results of the field project in two Ethiopian sites show how the Australopithecus' brains developed and how they were organized. Researchers scanned the fossil cranium using synchrotron microtomography in a facility in Grenoble, France. Through fossil reconstruction and the calculation of dental growth lines, the result yielded a remarkably preserved brain imprint of the Dikika child as well as other endocranial features of the Australopithecus fossils that were previously undetected.

Previous endocranial imprints show that the brain organization of Australopithecus afarensis is more ape-lie with no features derived towards humans. However, a comparison of adult and infant endocranial features indicates that the protracted brain growth in the Australopithecus species is more human-like. This prolonged brain development is likely critical for the evolution of extended childhood in hominins.

The researchers have discussed in their research that prolonged brain growth and maturation are commonly considered as consequences to the evolutionary increase in brain size in hominins. However, the data in the new study indicate that protracted brain development is not merely a result of the evolution and increase in brain size. Previously, the central tenet on the brain size of the newborn Australopithecus afarensis was hypothesized to be such due to obstetric constraints. However, the new findings challenge this view and suggest further that these obstetric challenges are not the proximate cause of the origins of prolonged brain growth in hominins.

Furthermore, the dental development of the Dikika child, as evidenced by its virtual dental histology, shows that it was faster than in modern-day humans. However, small relative endocranial volumes or rEV suggest a prolonged period of brain growth relative to chimpanzees. It demonstrates that the pace of teeth and brain development may not always coincide and that they can evolve independently of each other to some degree.

Continued brain development in hominins emerged more than three million years ago, and this could signify that subsequent hominin evolution followed a similar pattern. However, researchers also believe that patterns of brain development may have varied among the hominins and did not follow a linear evolution towards modern-day human conditions.

These new findings suggest that there were extended infant-care strategies among the Australopithecus afarensis. Primates, in general, adopt postnatal care, and protracted brain development in hominins may indicate that they have had a long dependence on caregivers, which is a basis for the evolution of the brain and social behavior.